Conventionally, thermoelectric generation of electricity is known in which the thermoelectric elements are disposed between a heat exchanger at a higher temperature part and another heat exchanger at a lower temperature part to generate electricity. The thermoelectric element is an application of a thermoelectric effect to be called Seebeck effect. In the case where a semiconductor material is used as a thermoelectric material, the thermoelectric power module is configured by electrically connecting a thermoelectric element (P-type element) formed of a P-type semiconductor thermoelectric material and another thermoelectric element (N-type element) formed of an N-type semiconductor thermoelectric material via an electrode.
Such a thermoelectric power module has a simple structure and can be easily treated, and stable characteristics can be retained. Therefore, research work thereof has been widely progressed toward application for the thermoelectric generation of electricity in which electricity is generated by utilizing heat in a gas discharged from an engine of a car, a furnace of a factory, and so on.
Generally, the thermoelectric power module is used in a temperature environment where a difference between a temperature (Th) at a higher temperature part and a temperature (Tc) at a lower temperature part becomes large in order to obtain high thermoelectric conversion efficiency. For example, a thermoelectric power module employing a typical bismuth-tellurium (Bi—Te) based thermoelectric material is used in a temperature environment where a temperature (Th) at the higher temperature part becomes 250° C. to 280° C. at maximum. Accordingly, diffusion of a material of a joint layer for joining the thermoelectric element to the electrode into the thermoelectric element or oxidation of the thermoelectric element becomes a problem.
As a related art, Japanese patent application publication JP-P2001-28462A (paragraphs 0001-0007) discloses a thermoelectric element which has a barrier film for preventing diffusion of an electrode material from the electrode, and conveyance of which at a conveyance process or the like in a manufacturing process is easy. JP-P2001-28462A is aimed to solve the problem that, in the case where the barrier film for preventing diffusion of an electrode material (Cu) from the electrode into the thermoelectric element is formed of Ni or a Ni-base alloy, the barrier film is magnetized to adhere to an apparatus or the barrier film adheres to the magnetized apparatus on the contrary at an assembly process, and thereby the manufacturing process is delayed.
It is described that the barrier film preferably consists of at least one kind of metal or alloy selected from a group consisting of Ag, Al, Cr, Mo, Pt, Pd, Ti, Ta, W, Zr, V, Nb, and In. However, in the case where a solder joint layer for improving joint with solder is provided between the solder and the barrier layer in order to join the thermoelectric element to the electrode, diffusion of a constituent element of the solder joint layer cannot be effectively prevented.
Further, in the case of a thermoelectric power module which is used in a temperature range not lower than 250° C. for example, the material itself of the barrier film diffuses into the thermoelectric element, and therefore, Ag, Cr, Pt, Pd, and In, a film of which can be formed by a plating method, cannot be used for a long time. On the other hand, as to Al, Mo, Ti, Ta, W, Zr, V, and Nb, a film of which cannot be formed by the plating method, a film thereof is generally formed by a PVD method such as a vapor deposition method. However, JP-P2001-28462A describes that a total thickness of a film formed by vapor deposition is 100 nm to 1000 nm, and if a film thickness exceeds 1000 nm, the film becomes apt to peel off a substrate due to stress in the film and cannot be used positively (paragraph 0027).
However, according to a result of investigation conducted by the inventors of the present application by using a molybdenum (Mo) film, no sufficient effect of preventing diffusion and oxidation can be obtained in the case where the film thickness is 1.34 μm or less. FIG. 25 shows difference of effects of preventing diffusion and oxidation according to a thickness of a molybdenum film formed on a surface of an N-type element. The thickness of the molybdenum (Mo) film in each sample was measured at three locations of each sample within one lot. FIG. 25 shows a result of an endurance test performed for three kinds of samples in the air at a temperature of 350° C. for 500 hours.
FIG. 25(A) shows a photomicrograph of a cross section of a sample in which a molybdenum film has a thickness of 0.25 μm to 0.36 μm, and FIG. 25(B) shows a photomicrograph of a cross section of a sample in which a molybdenum film has a thickness of 0.70 μm to 1.34 μm. It is understood that oxidation has progressed within the N-type element in both samples. On the other hand, FIG. 25(C) shows a photomicrograph of a cross section of a sample in which a molybdenum film has a thickness of 4.08 μm to 5.34 μm. It is understood that oxidation within the N-type element is suppressed in this sample. Further, according to a film forming technique used by the inventors of the present application, even in the case where the film thickness is 3 μm or larger, peeling off of the molybdenum film does not occur for a long time.
Japanese patent application publication JP-A-H9-293906 (paragraphs 0004-0006) discloses that Cu of an electrode diffuses into a semiconductor via solder at a high temperature in the case where the solder at a high temperature is used, and discloses a thermoelectric element aimed to prevent a decline of a thermoelectric conversion efficiency of the semiconductor itself due to the diffusion. The thermoelectric element is characterized in that an interposing layer in contact with a Bi—Te based semiconductor having a conducting type of P-type or N-type is connected to the electrode, and the interposing layer is one of a group consisting of Al, Ti, and Mg, or an alloy thereof.
However, as a result of investigation conducted by the inventors of the present application, a film of Al or Ti cannot be formed by the plating method, and therefore, the film is generally formed by using a sputtering method or a vapor deposition method as a thin film technology, or a screen printing method as a thick film technology. In the case where the film thickness is set to several micrometers or larger in a conventional thin film technology, peeling off occurs due to difference between linear expansion coefficients of the film and the thermoelectric material, and therefore, it is difficult to repetitively change a temperature for a long time. On the other hand, a film formed by a thick film technology lacks elaborateness, and therefore, there is a problem that a semiconductor directly under the film is oxidized due to transit of oxygen at a high temperature of, for example, 250° C. or higher, and the electric resistance increases.
Further, according to a result of investigation conducted by the inventors of the present application, when a thermoelectric element formed with sputtered films of titanium (Ti) and nickel (Ni) is heated to a temperature of 350° C., mutual diffusion of materials occurs between the nickel film and the thermoelectric element, and nickel is oxidized in the thermoelectric element directly under the titanium film, and thus, the above-mentioned purpose cannot be achieved.
FIG. 26 shows a change in an endurance test in the case where sputtered films of titanium and nickel are formed in sequence on a surface of a P-type element. FIG. 26(A) shows a photomicrograph of a cross section of a sample in which a titanium (Ti) film having a thickness of 0.5 μm and a nickel (Ni) film having a thickness of 0.5 μm are formed in sequence on the surface of the P-type element. Further, FIG. 26(B) shows a photomicrograph of the cross section of the sample after the endurance test is performed in the air at a temperature of 350° C. for 500 hours. It is understood that materials (Ni, Te, and so on) have mutually diffused between the nickel film and the P-type element, and oxidation has occurred in a portion of the P-type element.
FIG. 27 shows a change in an endurance test in the case where sputtered films of titanium and nickel are formed in sequence on a surface of an N-type element. FIG. 27(A) shows a photomicrograph of a cross section of a sample in which a titanium (Ti) film having a thickness of 0.5 μm and a nickel (Ni) film having a thickness of 0.5 μm are formed in sequence on the surface of the N-type element. Further, FIG. 27(B) shows a photomicrograph of the cross section of the sample after the endurance test is performed in the air at a temperature of 350° C. for 500 hours. It is understood that materials (Ni, Te, and so on) have mutually diffused between the nickel film and the N-type element, and oxidation has occurred in a wide range of the N-type element.
Japanese patent application publication JP-P2006-147600A (paragraphs 0023-0024) discloses obtaining a thermoelectric module which has high efficiency especially at a middle or high temperature such as 400° C. or higher, and in which aged deterioration and performance decrement very hardly occur. The thermoelectric module consists of a thermoelectric conversion part, a heat absorption part, and a heat radiation part, and is characterized in that the thermoelectric conversion part and the heat absorption part are firmly fixed to each other via a stress relaxation layer into one body.
In the thermoelectric module disclosed in JP-P2006-147600A, a metal foil (Cu, Fe, Ni, Ag, Ti, Zr, Al, Nb, Mo, or the like) occluding hydrogen is sandwiched between a thermoelectric element and an electrode in order to join the thermoelectric element to the electrode without using any inclusion such as a special jointing material or a sprayed layer, or flux (See paragraph 0044).
However, in the case where the thermoelectric module is used at a high temperature of, for example, 250° C. or higher, there is a problem that Cu, Fe, Ni, and Ag easily diffuse into the thermoelectric element to deteriorate the thermoelectric conversion characteristics. On the other hand, Ti, Zr, Al, Nb, and Mo hardly diffuse into the thermoelectric element, but have linear expansion coefficients greatly different from that of a thermoelectric material. Accordingly, in the case where the electrode and the thermoelectric element are joined to each other without using solder, there is a high possibility of damage if a temperature is repetitively changed for a long time. Further, the metal foil occluding hydrogen is undesirable in view of safety and cost.
Japanese patent application publication JP-A-H11-186616 (paragraphs 0004 and 0015) discloses a thermoelectric element in which an alloy layer is formed on a thermoelectric semiconductor in order not to degrade the performance, and thereby deterioration of the thermoelectric element can be prevented when an electrode is joined thereto and when an electric current flows after the electrode is joined. The thermoelectric element consists of (a) a thermoelectric semiconductor of a Bi—Te—Se base alloy (n-type) or a thermoelectric semiconductor of a Bi—Sb—Te base alloy (p-type), (b) an alloy layer of at least one kind of element of trivalent or tetravalent elements (B, Ga, In, Tl, C, Si, Ge, and Sn) and at least one kind of metal of Si, Sb, Te, and Se, or a Bi—Te—Se base alloy, or a Bi—Sb—Te base alloy, (c) a layer consisting of at least one kind of element of the trivalent or tetravalent elements (B, Ga, In, Tl, C, Si, Ge, and Sn), (d) a layer consisting of at least one kind of element of metals (Ti, Cr, Co, Ni, Nb, Mo, and W) having a diffusion preventing effect, and (e) electrode materials (a solder material and an electrode).
JP-A-H11-186616 is aimed to prevent diffusion of a material of the electrode into the thermoelectric semiconductor. However, since the diffusion prevention layer is disposed on the layer of the trivalent or tetravalent element (See FIG. 4 of JP-A-H11-186616), it is impossible to sufficiently prevent diffusion of the trivalent or tetravalent element into the thermoelectric semiconductor. Especially, Ga, In, Ge, and Sn are easily dissolved into the thermoelectric material and function as an acceptor, and therefore, it is difficult to maintain the alloy layer of at least one kind of element thereof and the at least one kind of metal of Bi, Sb, Te, and Se, or the Bi—Te—Se base alloy, or the Bi—Sb—Te base alloy to be stable even at a high temperature. Thus, there is a problem that thermoelectric conversion characteristics of the thermoelectric material are easily deteriorated.
Further, as the metals having a diffusion preventing effect, Ti, Cr, Co, Ni, Nb, Mo, and W are exemplified. However, as to Ti, Nb, Mo, and W, a film thereof cannot be formed by a plating method, and therefore, the above-mentioned problem occurs unless special contrivance is made, and it is difficult to obtain a sufficient diffusion preventing effect at a high temperature of, for example, 250° C. or higher. Furthermore, Co, Ni, Cr, and so on are apt to diffuse into the thermoelectric material, and form an alloy or intermetallic compound with Te in some cases to deteriorate thermoelectric conversion characteristics, and therefore, they are not much suitable.
Japanese patent application publication JP-P2008-10612A (paragraphs 0010-0012) discloses a method of manufacturing a thermoelectric element, which method is capable of forming a diffusion prevention layer effective for preventing diffusion of elements and having a high peel strength, on a thermoelectric material containing at least one of bismuth, tellurium, selenium, and antimony, and discloses a thermoelectric element manufactured by using such a method of manufacturing a thermoelectric element. The thermoelectric element includes a thermoelectric material containing at least two of bismuth (Bi), tellurium (Te), selenium (Se), and antimony (Sb), a diffusion prevention layer formed on the thermoelectric material and for preventing diffusion of a different kind of element into the thermoelectric material, and a solder joint layer formed on the diffusion prevention layer and for joining the diffusion prevention layer and solder to each other, and is characterized in that a peel strength at an interface between the thermoelectric material layer and the diffusion prevention layer or an interface between the diffusion prevention layer and the solder joint layer is 0.6 MPa or more.
The thermoelectric element disclosed in JP-P2008-10612A has a structure of an electrode/a solder layer/a solder joint layer/a diffusion prevention layer/a thermoelectric material layer, and thereby, greatly improves the problem in JP-P2001-28462A, JP-A-H9-293906, JP-P2006-147600A, and JP-A-H11-186616. However, it is insufficient in view of preventing mutual diffusion between the solder layer or the older joint layer and the thermoelectric material layer, or preventing oxidation of the thermoelectric material layer.
FIG. 28 shows a result of an endurance test in the case where a molybdenum film, a nickel film, and a tin film are formed in sequence on a surface of a thermoelectric material layer. In this thermoelectric power module, a molybdenum (Mo) film having a thickness of 5 μm, a nickel (Ni) film having a thickness of 1 μm, and a tin (Sn) film having a thickness of 0.2 μm are formed in sequence on the surface of the thermoelectric material layer. The tin (Sn) film is joined to an electrode via a solder layer. FIG. 28 shows a photomicrograph of a cross section of a thermoelectric power module after an endurance test is performed in the air at a temperature of 280° C. for 5000 hours. It is understood that nickel (Ni) has diffused into the thermoelectric material layer and oxidation occurs in a portion of the thermoelectric material layer, and also the thermoelectric material (Te) has diffused into the solder layer.